Hi cjeam and yvmuell,
I have a very similar problem as cjeam. Interestingly, my problem also started early November with a Fairphone adapter that otherwise worked very well for many months (and that I took incredibly good care of), so I am still suspicious this could be an issue due to one of the latest system updates (after all, I had another problem of “ghost typing” that also appeared after a software update - and thankfully was solved a couple of months later with another).
In my case I only have an USB audio adapter from Fairphone (so it doesn’t charge). When I ever so slighly move the phone the phone tells me I am now charging the device and stops whatever video/music I am watching/listening: if I click play then the sound comes out of the phone speakers. I tried not moving with the phone but it honestly makes no sense: I can’t listen to music walking in the street and have to stay completely immobile to enjoy music/videos. Sometimes I have to unplug-re-plug several times the adapter until the system stops thinking it’s a charger.
I will now try and buy another adapter as recommended by yvmuell, but I also want to share my frustration with this issue, one of many since I bought my fairphone less than a year ago. I have spent several hours trying to get to the bottom of this issue and should I want an adapter on yvmuell list I would have to go to a shop that is far from my home (as I don’t buy from amazon which I think is relatable to many fairphone users). I’ll buy one from another brand and will keep you posted if it works and solves the problem 
For what it’s worth, thank you both for posting and yvmuell for the list. I’m annoyed at all the reading and self-repairing I have to do with this phone but I am at least glad that most of my issues were somewhat solvable and for the help I got from this forum. Hope it’s the same with this one as well
I wish you both a nice week
Chiara